Cannabidiol or CBD is one of the many cannabinoid compounds found in Cannabis sativa (commonly known as marijuana). Unlike other cannabinoid compounds like the psychoactive tetrahydrocannabinol or THC, CBD does not cause adverse effects on humans; or more accurately, adverse effects may be caused due to drug-to-drug interactions between CBD and other medications received by the patient.
According to a Pre-Review report by the World Health Organization in 2017, CBD has low abuse potential and no effects indicative of dependence; making it a substance of a good safety profile.
The effectiveness is, of course, dependent on a number of factors including the bioavailability of CBD.
What is Bioavailability?
Bioavailability refers to the amount or percentage of unchanged substance that is able to reach the circulatory system or target organ to achieve the desired effects. Depending on how the drug is administered, it can be broken down or be significantly reduced. When prescribing dosage and route of administration, doctors have already considered the bioavailability of the medicine.
Several studies have reported on the pharmacokinetics and the resulting bioavailability of cannabinoids as a group, with some specifically targeting THC and its relation to CBD and cannabinol (CBN).
But there has yet to be a study that deals with the bioavailability of CBD alone, although it has been cited that THC and CBD have similar rates. As such, the CBD bioavailability listed below deals with cannabinoids in general unless otherwise stated. Also, for some routes of administration, only test animals were used and listed below those with human subjects.
Another factor that plays a role in bioavailability of CBD is the vehicle or the form of CBD that is taken by the patient. CBD can come in the form of capsules, edibles, oil concentrates and tinctures, sprays and even beverages.
Routes of Administration
Bioavailability depends heavily on the dosage and route of administration also known as how the drug is taken. There are several studies regarding the intake and absorption of CBD among them are:
Oral Consumption generally at 6%
CBD can be taken orally in a number of ways. The traditional method is the ingestion of the product.
CBD edibles like cake pops or gummies are eaten and digested. Unfortunately, this produces the lowest bioavailability of CBD due to the “first pass elimination” or the process a substance undergoes from the site of administration to the target site.
The CBD product, when ingested, goes through the digestive system and passes through the liver causing it to be heavily broken down (or metabolized), leaving only a fraction of the unchanged substance to enter the bloodstream.
20mg of THC in a cookie yielded an oral bioavailability of 6% (Ohlsson et al. 1980) but generally ranges from 4-20%. Oral consumption of THC and CBD were found to be the same at 6% (Karschner, Erin L, et al. 2010).
It has been reported that mixing the CBD with oils or fats help in countering the degradation due to the first pass metabolism. In fact, a new formulation, capsule form PTL401, resulted in higher plasma concentrates and “bioavailability was also higher (131% and 116% for CBD and THC, respectively)compared to the standard Sativex spray (Atsmon J. et al. 2018).
Oromucosal dosing generally at 35%
Due to the low bioavailability of CBD through oral consumption, the alternative oromucosal method is available. CBD forms like tinctures, sprays, lozenges, and concentrates are used for this route. Nabidiolex and Sativex are some of the standardized extract preparations for CBD for oromucosal dosing.
Sublingual dosing involves placing the product under the tongue, wherein it is absorbed by the mucous membrane and would be diffused into the bloodstream via the capillaries in the connective tissue.
Buccal administration is similar to sublingual wherein the substance is rubbed or placed between the gums and cheeks where it will diffuse and enter the bloodstream.
Although the general bioavailability of CBD and other cannabinoids through oromucosal dosing is at 35%, a study by Karschner, Erin L et al. (2010) show that a low dose of Sativex 5mg THC had a bioavailability at 92.6% (13.1%), and 15mg THC high dose of Sativex yielded 98.8% (11.0%).
Inhalation ranges from 2-56%
Aerosolized CBD has been reported to have higher bioavailability than oral consumption of the same, based on the WHO report stated above.
Smoking makes bioavailability of CBD range from 2-56% depending on the smoking dynamics of the smoker: the number of puffs, inhalation volume, hold time, etc. (Huestis 2007).
Vaporizing is another method being studied. A volcano vaporizer having 200mg of CBD yields an availability of 40% in vapor phase inside a balloon (Solowij, Nadia et al. 2014). No tests have been made on living subjects yet.
Rectal approximately 12%
Due to the unpopularity of suppositories, there are few studies regarding bioavailability of cannabinoids through rectal administration. One study notes that rectal THC was found to be twice bioavailable in subjects compared to the oral THC (IJ McGilveray. 2005).
1. Injections: (IM) Intramuscular (IV) Intravenous
Intravenous administration, since injected straight into the veins, is considered to have 100% bioavailability but is not a common way to administer CBD. It is mostly used to compare the effectiveness of other routes.
2. Topical Means (rats and guinea pigs)
Transdermal or transcutaneous routes are also possible for CBD. A study on rats included CBD gel at 1% and 10% concentrations, respectively (Hammell et al. 2016). Although able to raise the amount of CBD to the immediate area, there is no significant data to attest to the degree of bioavailability of CBD through transdermal means.
Another study on guinea pigs supports the transdermal application of CBD gel for chronic pain treatment (Paudel, KS et al. 2010).
3. Intranasal (rats)
Intranasally administered CBD had a bioavailability of 34-46% (Paudel, KS et al. 2010).
Clearly, more research is required to determine the bioavailability of CBD in human patients, but hopefully, this has been an informative list for everyone interested in the effectiveness of cannabinoids as treatments.